The present invention relates to light-emitting devices and in particular organic light-emitting devices (OLEDs). In particular, the invention relates to emitter materials in which charged metal complexes are bonded to a polymer by electrostatic interactions.
In recent years, a novel technology based on the use of electroluminescent materials in so-called OLEDs (organic light-emitting diodes) has been developed in the area of display and lighting technology. The first OLEDs were developed in 1987 (Tang, C. W. et al., Appl. Phys. Let. 51, 913 (1987)). The way in which OLEDs function is based on a multilayered structure which comprises an emitter layer, a hole-conduction layer and an electron-conductor layer. The layers consist predominantly of organic substrates, which can be made very thin and flexible. OLED devices can be manufactured with large surface areas as lighting elements or display screens, but also in the form of smaller displays.
In the production of OLEDs, the various organic layers are applied to a support material. To this end, two different techniques are essentially employed. In vacuum evaporation, molecules are vapour-deposited in vacuo. In a wet-chemical process, the layers are applied from a solution, for example by spin coating, ink-jet printing, doctor blading or screen-printing processes.
The way in which OLEDs function has been described in detail, for example, in H. Yersin, Top. Curr. Chem. 2004, 241. An overview of the function of OLEDs is also given in C. Adachi et al., Appl. Phys. Lett. 2001, 78, 1622; X. H. Yang et al., Appl. Phys. Lett., 2004, 84, 2476; J. Shinar, “Organic light-emitting devices—A survey”, AIP-Press, Springer, New York, 2004; W. Sotoyama et al., Appl. Phys. Lett. 2005, 86, 153505; S. Okada et al., Dalton Trans. 2005, 1583 and in Y.-L. Tung et al., Y. Mater. Chem. 2005, 15, 460-464. An up-to-date review of the state of the art is given, for example, in “Highly Efficient OLEDs with Phosphorescent Materials”, ed. H. Yersin, Wiley-VCH, Weinheim, Germany 2007.
Since the first reports of OLEDs, these devices and the materials used therein have been intensively developed further. In particular, the emitter materials employed are currently the subject of intensive research.
In particular, so-called triplet or phosphorescent emitters have recently become the focus of research. It has been found that a significantly greater electroluminescence quantum yield can be achieved with phosphorescent emitters than with so-called singlet emitters. Whereas only a transition from the excited singlet state into the singlet ground state results in emission of light in the case of singlet emitters (purely organic compounds), higher electroluminescence quantum yields are possible in the case of organometallic complexes since here light is emitted on transfer from the excited triplet state. Triplet emitters are described, for example, in WO 2004/017042 A2 (Thompson), WO 2004/016711 A1 (Thompson), WO 03/095587 (Tsuboyama), US 2003/0205707 (Chi-Ming Che), US 2002/0179885 (Chi-Ming Che), US 2003/186080 A1 (J. Kamatani), DE 103 50 606 A1 (Stöβel), DE 103 38 550 (Bold), DE 103 58 665 A1 (Lennartz), WO 2007/118671 (Yersin). Higher electroluminescence quantum yields are also achieved using phosphorescent lanthanoid complexes.
However, the known emitter materials have various disadvantages. For example, the low thermal stability and the chemical stability to water and oxygen are problematical. In addition, many emitter materials have an excessively short lifetime for use in high-quality electronic applications. There is also a further need for improvement with respect to good synthetic accessibility and manufacturing reproducibility.
A very large number of charged emitter molecules is known which have extraordinarily high emission quantum yields and in which many of the above-mentioned disadvantages do not occur. However, the bonding of charged emitters into an emitter layer of an OLED device causes problems. Owing to their lack of volatility, charged metal complexes cannot be applied by vacuum evaporation. In the case of wet-chemical application, crystallisation/salt formation causes problems. Migration of the ions in the electric field results in different potential ratios in the OLED device.
Starting from the outlined problems of the prior art and the highly promising potential of charged emitters, the object of the present invention was to make charged emitters and in particular charged phosphorescent or triplet emitter metal complexes usable for use in OLED devices.
To this end, it is necessary to restrict the mobility of the charged emitters. Immobilisation of the emitters can be achieved by bonding to a polymer. The strategy of bonding complexes covalently to polymers is described, for example, in P. K. Ng et al., Chem. Eur. J. 2001, 7, 4358; X. Chen et al., J. Am. Chem. Soc. 2003, 125, 636 and in J. Hjelm et al., Inorg. Chem. 2005, 44, 1073. However, the synthetic accessibility of phosphorescent polymers with covalently bonded triplet emitters has hitherto only been possible by very complex methods. The materials can only be obtained in multistep synthetic processes and frequently in unsatisfactory yields.